Vehicle suspension systems are well known in the art. Such suspension systems have as their goal the control of the relative motion between the sprung (chassis) mass and unsprung (associated tire) mass of the vehicle. Suspension systems are classified as either passive, semi-active, or active.
Passive suspension systems dissipate energy produced when a vehicle is driven over an irregular road surface. Such systems provide good vibration isolation. A linear response of a passive suspension system can be altered by (i) adding an advantageous nonlinear attribute, such as direction dependant damping, and (ii) minimizing an objectionable effect, such as stiction. Passive systems, however, react only to applied forces from below through the road surface and from above through inertia of the sprung mass or vehicle body. Ideally, a suspension system should appear "soft" in reacting to road noise inputs and stiff when reacting to inertia inputs. Since a passive system cannot distinguish between the two, an engineering compromise is made.
An active suspension system uses power from the vehicle engine to actively move the vehicle wheels over an irregular road surface. Rather than a shock absorber, as is found in passive suspension system, an active suspension system uses a hydraulic servo-actuator, i.e., a hydraulic motor, to move the vehicle wheel. A plurality of sensors are located at various vehicle locations. A controller, e.g., a microcomputer, monitors the sensor outputs and controls operation of the hydraulic actuator through an electrically controlled servo valve. Through a control algorithm, the controller controls reaction to road noise and inertia inputs and controls relative motion of the sprung and unsprung masses.
In an active suspension system, the servo valve and controller function as an energy control device. The servo valve connects the energy source, i.e., a pump, to the energy consumer, i.e., an actuator. The difference between power in and power out is converted to heat energy by the servo valve.
In a fully active suspension system, the actuator is operated so as to move the wheel up and down relative to the vehicle body as necessary to provide a desired "ride feel" and "handling characteristic" of the vehicle. The hydraulic pump provides energy in terms of fluid flow at system pressure. The servo valve removes energy at a rate to provide fluid flow and pressure so as to move the wheel at a velocity needed to achieve the desired ride feel and handling characteristics. Control of fluid flow controls actuator velocity. Control of fluid pressure controls actuator force.
In a typical fully active suspension system, each corner i.e., each wheel, of the vehicle has an associated actuator and servo valve. The power consumption of each corner is the product of the fluid flow to the actuator times the supply pressure. Road noise occurs at high frequencies. Large strut velocities are often required to prevent large inputs from effecting commensurate motion of the vehicle chassis. Such large velocities requires that a large amount of energy. Since the hydraulic pump is driven by the vehicle engine, a large amount of energy consumed by the active suspension system means that a large amount of energy is being drained from the vehicle engine.
It is therefore desirable to develop a suspension system that provides better ride and handling control than a passive system but does not consume the energy required by a fully active suspension system.